Aircraft Construction

and Columbia line of production aircraft, leading to their high performance despite their fixed landing gear. Composites also help mask the radar signature of “stealth” aircraft designs, such as the B-2 and the F-22. Today, composites can be found in aircraft as varied as gliders to most new helicopters. Lack of corrosion is a third advantage of composites. Boeing is designing the 787, with its all-composite fuselage, to have both a higher pressure differential and higher humidity in the cabin than previous airliners. Engineers are no longer as concerned about corrosion from moisture condensation on the hidden areas of the fuselage skins, such as behind insulation blankets. This should lead to lower long-term maintenance costs for the airlines. Figure 3-15. Composite aircraft. Another advantage of composites is their good performance in a flexing environment, such as in helicopter rotor blades. The most common reinforcing fibers used in aircraft Composites do not suffer from metal fatigue and crack growth construction are fiberglass and carbon fiber. Fiberglass as do metals. While it takes careful engineering, composite has good tensile and compressive strength, good impact rotor blades can have considerably higher design lives than resistance, is easy to work with, and is relatively inexpensive metal blades, and most new large helicopter designs have all and readily available. Its main disadvantage is that it is composite blades, and in many cases, composite rotor hubs. somewhat heavy, and it is difficult to make a fiberglass load- carrying structure lighter than a well designed equivalent Disadvantages of Composites aluminum structure. Composite construction comes with its own set of disadvantages, the most important of which is the lack of Carbon fiber is generally stronger in tensile and compressive visual proof of damage. Composites respond differently from strength than fiberglass and has much higher bending other structural materials to impact, and there is often no stiffness. It is also considerably lighter than fiberglass. obvious sign of damage. For example, if a car backs into an However, it is relatively poor in impact resistance; the fibers aluminum fuselage, it might dent the fuselage. If the fuselage are brittle and tend to shatter under sharp impact. This can is not dented, there is no damage. If the fuselage is dented, be greatly improved with a “toughened” epoxy resin system, the damage is visible and repairs are made. as used in the Boeing 787 horizontal and vertical stabilizers. Carbon fiber is more expensive than fiberglass, but the price In a composite structure, a low energy impact, such as a has dropped due to innovations driven by the B-2 program bump or a tool drop, may not leave any visible sign of the in the 1980s and Boeing 777 work in the 1990s. Very well- impact on the surface. Underneath the impact site there may designed carbon fiber structures can be significantly lighter be extensive delaminations, spreading in a cone-shaped area than an equivalent aluminum structure, sometimes by 30 from the impact location. The damage on the backside of percent or so. the structure can be significant and extensive, but it may be hidden from view. Anytime one has reason to think there Advantages of Composites may have been an impact, even a minor one, it is best to Composite construction offers several advantages over get an inspector familiar with composites to examine the metal, wood, or fabric, with its lighter weight being the most structure to determine underlying damage. The appearance frequently cited. Lighter weight is not always automatic. It of “whitish” areas in a fiberglass structure is a good tip-off must be remembered that building an aircraft structure out of that delaminations of fiber fracture has occurred. composites does not guarantee it will be lighter; it depends on the structure, as well as the type of composite being used. A medium energy impact (perhaps the car backing into the structure) results in local crushing of the surface, which A more important advantage is that a very smooth, compound should be visible to the eye. The damaged area is larger than curved, aerodynamic structure made from composites the visible crushed area and will need to be repaired. A high reduces drag. This is the main reason sailplane designers energy impact, such as a bird strike or hail while in flight, switched from metal and wood to composites in the 1960s. results in a puncture and a severely damaged structure. In In aircraft, the use of composites reduces drag for the Cirrus 3-10